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Comments from LCLS FAC Meeting (April 2004): J. Rößbach:“How do you detect weak FEL power when the gain is very low (few hundred)?” K. Robinson:“Can you modulate the laser heater in some way to help FEL signal detection?” Comments from LCLS FAC Meeting (April 2004): J. Rößbach:“How do you detect weak FEL power when the gain is very low (few hundred)?” K. Robinson:“Can you modulate the laser heater in some way to help FEL signal detection?” Laser-Heater Modulation P. Emma, Z. Huang, J. Wu

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modulate laser power (1.2 to 19 MW @ 7 Hz) Kem Robinson idea… modulated slice energy spread (0.01% to 0.04% rms at 14 GeV) laser heater Laser Heater at 135 MeV

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Simulate LCLS (Linac + M. Xie) with linac jitter, large emittance (3 m), spontaneous radiation background, and laser-heater modulated at 7 Hz (or other unique frequency << 120 Hz) 1.5-Å LCLS, except x,y = 3 m 2% rms charge jitter 0.5 ps rms gun-timing jitter 0.1-deg rms RF phase jitter (each of 4 linacs) 0.1% rms RF amplitude jitter (each of 4 linacs) 2% rms emittance jitter includes small emittance growth in laser-heater 100-MW spontaneous power (1% BW cut)* 10% rms radiation energy measurement noise fast, accurate 2 nd -order linac model in Matlab, with jitter * Sven Reiche says 10-MW at 1% BW cut (allows a 3.5- m emittance)

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Effects of Linac Jitter bunch length energy spread peak current e relative energy bunch arrival time

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P < P sat : P > P sat : 1-D startup power: Use ‘Ming Xie’ model to calculate gain length, L g, then calculate power as: Assume 1% BW cut to get 100-MW spontaneous power (none)

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Red:Total power + meas. noise Magenta:Spontaneous power Green:FEL power Gain 230

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14.7 ± 4.1 m 0.029 ± 0.013 % x,y = 3.0 m 4.2 ± 0.6 kA 0 ± 0.088 % 1.0 ± 0.02 nC 21 ± 3.0 m

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7 Hz FFT of total power + noise Gain 230 Get similar result if P spont = 10 MW and x,y = 3.5 m

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